We report studies on vascular endothelial function and ultrastructure in 2 cases of fatal cyclosporin (CS)-associated thrombotic microangiography following allogeneic bone marrow transplantation (BMT). Spontaneous vascular release of prostacyclin (PGI2) from a vein sample ex vivo was absent, and scanning electron microscopy (SEM) showed surface changes indicative of vascular endothelial damage (case 1). PGI2 release from cultured human umbilical vein endothelial cells incubated with patients' serum in vitro was normal in both cases. Plasma von Willebrand factor (vWF) antigen and ristocetin cofactor activity levels were raised in both patients, 5.06 and 7.02 (case 1) and 3.60 and 2.01 (case 2) (normal ranges 0.59-1.57 and 0.42-1.74 U/ml), respectively, but multimer patterns were normal. The SEM appearances coupled with the absent PGI2 release and raised vWF levels suggest that vascular endothelial damage is central to the pathogenic process in thrombotic microangiopathy following allogeneic BMT but the mechanisms appear to be distinct from those in the haemolytic uraemic syndrome and de novo thrombotic thrombocytopenic purpura. The precise role of CS in this process remains to be identified.